The present invention is generally directed to an apparatus and processes thereof for the preparation of particulate materials with narrow particle size distribution properties. More specifically, the present invention relates to improved classifier chamber geometries, such as a height level profile, and which profiles enable a high level of control over the physics of the separation process in the classifier and thereby provide a constant cut point in the free vortex region of the classifier.
In particle processing arts, for example, for the preparation of fine and uniformly disperse particulate materials, there exists various equipment and mechanical processes for achieving selective separation of particulate powders into eligible and non-eligible particle size fractions or ranges, and are collectively referred to as classifiers and classification.
In the manufacture of particulate powders, such as electrostatographic toner compositions, a classifier apparatus employing a rotating wheel is commonly used to accomplish classification. In general, the rapidly rotating classifier wheel creates a dynamical fluid vortex which provides the necessary forces to achieve separation of particles greater than a certain size from particles less than a certain size.
The extent or sharpness of the separation of particles of different sizes achieved by the classifier is an important measure of the quality of the separation equipment and process, and is generally reflected in the quality of the resultant particles, for example, the physical performance characteristics and properties of the particles. The sharpness of the separation is also a measure of how well the classifier can discriminate among similarly sized particles. Ideally, a classifier will separate a feed particle stream containing a mixture of fine and coarse particles sizes into two distinct streams: a coarse stream and a fines stream with little or no overlap in size distribution.
The degree of sharpness of the separation is measured using a coarse grade efficiency calculation. The calculation indicates what fraction of particles with a certain size will travel to the coarse stream, and what fraction will travel to the fines stream. A ratio of the size at which 25 percent of the particles travel to the coarse stream (D.sub.25) and the size at which 75 percent of the particles travel to the coarse stream (D.sub.75) is used as a nominal measure of sharpness(D.sub.25 /D.sub.75). An ideal separation provides a sharpness (D.sub.25 /D.sub.75) equal to 1. In currently available commercial classification equipment, a sharpness index exceeding a value of 0.7, for example, from about 0.7 to about 1.0, is considered to be excellent and considered difficult to attain without exceptional effort and operating conditions.
Commercially available classifier wheels generally provide little or no profiling, or only provide a profile which maintains a constant wheel height or constant air flow radial velocity. These conditions typically result in a particle cut point situation which diminishes towards the particle outlet, and is believed to lead to an undesirable buildup of solids concentration in the free vortex region.